use Untitled instead of Untitled0

Various adjustments to ContentsManager.increment_filename:

- start with no number (Untitled0 -> Untitled.ipynb), copy still starts with 1
- copy of copy increments instead of adding another `-CopyX` (copy Foo-Copy1.ipynb gives Foo-Copy2.ipynb, not Foo-Copy1-Copy1.ipynb)
- copy file to new folder starts with the original filename, instead of unconditional `-Copy0`